The invention concerns a method of controlling the machining of a workpiece which is moving during the machining operation and on which a plurality of tool units operate simultaneously at different machining locations.
A machining operation, in particular a cutting machining operation on a metal workpiece, can be greatly influenced by varying the motion parameters of the workpiece and the tool, in particular the motion parameters relative to each other. In that respect, the endeavour is to achieve a given optimisation aim or target in the machining procedure, for example achieving a surface quality which is as high as possible, achieving an overall machining time which is as short as possible for each workpiece, or achieving the longest possible service life for the tools. The latter is nowadays the most frequent optimisation target, in consideration of the primary target of reducing costs.
Hereinafter reference is specifically made to machining a crankshaft by means of rotary or turning milling, in which a milling tool which rotates relatively quickly machines peripheral surfaces and/or end faces on a crankshaft which rotates relatively slowly, without however the present invention being limited thereto.
In cutting machining operations such as rotary milling, in particular high-speed rotary milling. optimisation of the machining procedure is governed not only in accordance with the cutting speed, that is to say the relative speed of the tool cutting edge relative to the workpiece at the respective machining location, but also a plurality of other factors such as temperature at the machining location, maximum and average depth of cut, spacing in respect of time between a cutting edge coming out of engagement and fresh engagement of the next cutting edge of the same tool, and so forth.
As two or more milling units which are controllable independently of each other in respect of their motion parameters and in particular their speed of rotation operate simultaneously on the rotating crankshaft at for example different axial positions, it is not possible to set all machining locations in themselves respectively to 100% of the possible optimisation target, as that is prevented by virtue of the speed of crankshaft rotation which can admittedly be freely selected but which is the same for all machining locations.
In accordance with DE 195 46 197 C1 therefore it is proposed that the motion parameters and in particular the speed of rotation of the crankshaft are to be so selected that 100% optimisation of machining is achieved at one of the machining locations. The resultant speed of crankshaft rotation has to be accepted for the other machining locations as an unavoidable input parameter so that these other machining locations can no longer achieve a 100% degree of optimisation.
The speed of crankshaft rotation therefore represents the master setting for the other machining locations, on the basis of the master-slave principle.
The disadvantage of that method however is that it provides that machining can admittedly be implemented in the optimum fashion for one of the machining locations, but it is not possible to achieve optimisation of the machining result, over the sum of the machining locations.
Accordingly the object of the invention is to provide a method of controlling such a machining procedure, which permits optimisation over the entire machining operation.
That object is attained by the characterizing features of the invention. Advantageous embodiments are set forth in the appendant claims.
That procedure admittedly means that the motion parameters, in particular the speed of crankshaft rotation, involve values which differ from the respective corresponding optimum values in respect of the individual machining locations, but on the other hand optimum machining is made possible in terms of the overall effect of the machining procedure.
It is only by departing from the motion parameters and in particular the speed of crankshaft rotation which would be the optimum value for a single one of the plurality of machining locations in operation at the same time, that it is possible to achieve an optimum machining result in terms of the overall effect thereof.
That can be verified on the basis of the following numerical example:
Looking at the crankshaft indicated at KW, high-speed external milling cutters a, b operate thereon at the two machining locations A, B. It will be assumed that the optimisation target is the longest possible service life for the milling cutters a and b respectively.
In accordance with the above-described known method, for example the machining location A would be 100% optimised, insofar as at that location, with the following parameters:
speed of crankshaft rotation nKW: 15 rpm, and
speed of rotation of the milling cutter na: 120 rpm, the cutter service life ta achieved is 1000 minutes.
Because the value of nKW=15 rpm, a relative optimisation effect is achieved at the machining location B by virtue of the fact that the milling cutter b rotates there at a value nb of 40 rpm, which provides a service life tb for that milling cutter b of 600 minutes.
If in contrast the speed of crankshaft rotation nKW were to be increased to for example 17 rpm, the speed of rotation na of the milling cutter a would therefore have to be raised to 140 rpm and that would involve a service life ta of only 950 minutes.
It will be noted however that, for the milling cutter b which would then rotate for example at a speed of 50 rpm, that could involve a service life of 700 minutes.
In overall total therefore the service life of the tools at 950+700=1650 minutes would be 50 minutes higher than with the previously known method which only afforded values of 1000+600=1600 minutes.
In addition, the shortest prevailing service life of the milling cutters a, b which are in operation at the same time is raised from 600 to 700 minutes, which means that, when beginning the machining procedure with two new milling cutters a, b, the first time a milling cutter has to be changed is only after 700 minutes and not after 600 minutes, and this could play an important part in terms of reducing the frequency of tool changes over the machine as a whole as the optimisation target.